Check how you're wearing your mei tai.
The mama who made my mei tai taught me how to put it on for a toddler. Made a HUGE difference.
Tie the waistband so that the bottom edge is ABOVE (not on) your hipbones. Cross the straps in back BEHIND (not under) your LO's butt, and do an extra twist in that cross. Then the straps will be pulling her up and in towards you.
Make sure that her knees are above her hips, she should be sitting well down in the mei tai.
Pictures here, second picture shows what to do:http://www.mothering.com/discussions....php?t=1224862
You'll be able to tell that you've got it right if you can easily put a hand between your shoulders and the straps.
My dd is now about 25lbs and I wore her for several hours at various times on moving day with no discomfort. When she was only 20lbs or so and I didn't know the good way to wear the mei tai, I couldn't even walk to the park (15 minutes) without excruciating pain.
And my mei tai has no special padding whatsoever. As with properly fitted bras, there should be no need for padding in the shoulder portion of a baby carrier, because the weight shouldn't be on your shoulders at all.